The Scott Specialized Catalog is an excellent resource, probably the best or most comprehensive for US stamps. (I have the 2013 edition; things change slowly enough in the stamp world that a year old copy, at half the cost of a new one, is good enough for me.) However, be careful using it as a "price guide." For all but the rarest or most desirable of stamps or covers, it seems to overstate the "value" by at least a third, if not more. My opinion here is based on having made hundreds of purchases on
ebay or Bidstart, and have observed countless more. I have a rule of thumb for bidding on
ebay, in which I will peg my high bid at 67% of CV (catalog values). I win most auctions I bid in following this rule of thumb, and at winning bids closer to 50%-55% of CV in most cases.
Also, being new to the hobby, do not think that these CV's represent what your collection might be worth selling to a dealer. CV, or even the lesser values I see on
ebay, represent "retail" value. You can only realize that selling yourself. A dealer is only going to offer a fraction of CV, since they have overhead costs of carrying inventory, and expect to make a profit when they resell what they buy from you.
Bottom line: having some idea of the value of a collection is useful, but collect stamps and philatelic material for enjoyments sake, and only secondarily, if at all, as "investment." In fact, again except for the rarest or most desirable items, stamps and covers are not particularly good investments. Most do not hold their value over time, i.e. do not appreciate much in value at all, and certainly not enough to keep pace with inflation.
Basil